Primary electrochemical cells are a class of voltaic cells. Voltaic cells are those electrochemical cells in which chemical changes produce electrical energy, in distinction to electrolysis cells in which electrical energy from an outside source produces chemical changes within the cell. Primary cells are those voltaic cells which cannot be conveniently recharged, which usually are discarded after a single exhaustion of their component elements, or which require replacement of their exhausted chemical constituents to bring them back to their original condition. These cells are distinguished from another class of voltaic cells, namely, secondary cells, in which the exhausted cell is charged by passing electrical current from an outside source through it in the reverse direction of the discharge current.
In a primary cell, chemical energy is converted to electrical energy with a reduction in the free energy of the system. In the course of the cell reaction, negative charge leaves the anode, travels through an external driven circuit, and re-enters the cell at the cathode. Thus, the cathode is the positive electrode and the anode is the negative electrode. By virtue of the established electromotive series, it is possible to select suitable cathodes and anodes to obtain a desired theoretical voltage. The ideal cell would give the theoretical voltage under continued, constant load and the loss in free energy would manifest itself entirely as electrical energy outside the cell. However, this ideal is never attained in practice, because the internal resistance of a cell is not zero and the reactions within the cell are never completely reversible. Moreover, problems of incompatibility of the cathode and anode with each other or with the electrolyte, polarization, and other well known problems prevent performance at theoretical values.
The demand for existing aqueous electrolyte primary electrochemical cells with actual discharge voltages of about 1.5 V is increasing rapidly with the popularity of cordless electrical entertainment, communications, and technical equipment. Currently, the major part of this demand is satisfied by two types of cells often referred to as the Leclanche or alkaline manganese dioxide/zinc cell. These cells use zinc anodes and manganese dioxide cathodes, wherein the cathode material is mixed with carbon to provide electronic conductivity. The electrolyte is in an aqueous solution of either ammonium chloride and zinc chloride or potassium hydroxide with potassium zincate. These aqueous cells suffer from the possibility of gas formation by reaction of the anode with the electrolyte and may not, therefore, be hermetically sealed. Further, the working voltage at constant load for these cells decreases steadily with the extent of discharge.
There have recently appeared in the literature a number of different electrochemical cells which employ lithium as the active anode material and which discharge at about 1.5 V. Lithium has several inherent advantages associated with its use, among which are high capacities and high specific energies coupled with a high degree of stability and long storage life times. Among these are the Li/CuO system (SAFT; G. Lehmann et al, 5th International Power Sources symposium, Brighton, 1974), the Li/PbCrO.sub.4 and Li/PbO.sub.2 systems (G. Pistoia et al, Electrochemica Acta, 22, pp. 1141-1145, 1977), the Li/Bi.sub.2 O.sub.3 system (Varta; U.S. Pat. No. 4,085,259), the Li/antimony oxide systems (Sb.sub.2 O.sub.3, SbO.sub.2 and Sb.sub.2 O.sub.5) (Varta; Ger. Offen. No. 2,516,703), and the Li/PbSiO.sub.3 and Li/CuAl.sub.2 O.sub.4 systems (Varta; Ger. Offen. No. 2,521,769).
The Pistoia reference further identifies some of the inherent advantages in the use of lead compounds which makes them worthy of further investigation as cathodes for lithium cells; however, there is no mention in this reference of a cell employing a lead sulfate cathode.